1. Field of Invention
The invention generally relates to tone detectors and, in particular, to a method for dual tone detection, implemented on a Digital Signal Processor (DSP) with aliasing bandpass filters, in the presence of speech, music and noise.
2. Description of Prior Art
Telephone communication systems commonly use a tone signal as a control command. The tone detection signals transfer call control information to a main communication network.
The dual frequency tone is a standard tone signal used in the public telephone system. For touch-tone dialing, the dual frequencies in the tone include a row component and a column component. The row and column refer to the location of the key on the grid of a telephone. For example, keys 1, 2, 3, and A share a row frequency, while keys 1, 4, 7, and * share a column frequency. For example, for digit 5, the phone sends row frequency 770 Hz and column frequency 1336 Hz. Most telephones do not have the fourth column, [A, B, C, and D].
This standard is set forth in Local Access and Transport Area Switching Systems Generic Requirements (LSSGR). These standards are referred to as Dual Tone Multi-Frequency Signaling (DTMF) which designates dual tone pairs having the frequencies as shown below in the DTMF row and column matrix:
TABLE 1 ______________________________________ 1 2 3 4 Row/Column 1209 Hz 1336 Hz 1477 Hz 1633 Hz ______________________________________ 1 697 Hz 1 2 3 A 2 770 Hz 4 5 6 B 3 852 Hz 7 8 9 C 4 941 Hz * 0 # D ______________________________________
Systems to detect dual tone pairs have been available since the advent of the field of Digital Signal Processing (DSP). The ability to perform tone detection in the analog domain has existed even longer. Tone detection involves the detection of tones from a set of known frequencies and declaring the detected tones valid by checking the minimum duration, the spectral energy surrounding the tones, the deviation of the tone frequencies from the expected frequencies and other elements per the industry standards.
Recent changes in telecommunications have placed increased requirements on the robustness of the tone detection and the capacity or number of channels simultaneously supported in a given amount of time (commonly referred to as "realtime") by the tone detector. The robustness and the capacity of a tone detector are inversely proportional values for a given DSP processing power. As the detector's noise immunity and rejection of digits simulated by speech or music increases, the capacity supported by the detector decreases substantially. Therefore, it is desirable to minimize the time required to perform tone detection on a single channel, while maximizing the robustness of tone detection.
One method for detecting a tone is to look at the entire spectral content of the signal on the channel by performing a Fast Fourier Transform (FFT) in an attempt to validate a DTMF tone pair. While effective, this approach is an expensive solution due to the DSP processing required to perform an FFT across the entire 4 khz speech band used in telephony.
A more viable alternative is to look for just the predefined tone frequencies using one of two well-known techniques: Infinite Impulse Response (IIR) Filters and Finite Impulse Response (FIR) Filters. IIR Filters are essentially filters that are a function of the signal and past filter outputs. FIR Filter outputs are based on only the current input signal.
TABLE 2 ______________________________________ LSSGR TR-NWT-000506 Issue 3, Echo: Accept Echo 20 ms Sept. 1991: Section 6.2. delayed LSSGR TR-NPL-000275 Issue 1, April and 16 db down 1986: Section II from orginal AT&T recommendations from Notes on signal the Network Section Single Frequency Deviation: -1.5% &lt; Accept &lt; + 1.5% Power: Accept &gt; = -3.5% &lt; Reject &lt; -25 dbm/freq +3.5% Reject &lt; -55 Other Specifications: dbm/freq Twist: -8 dbr &lt; Accept &lt; Talk.sub.-- off rejection +4 dbr from TR-TSY-000763 Tone On Duration: Accept &gt; = 40 ms Gaussian/Impulse noise rejection Reject &lt; 23 ms Third frequency rejection Minimum cycle time = Power line noise 93 ms rejection Interdigit Gap: Accept &gt; = 40 ms Registration in the presence of dial tone No minimum reject interval Rejection of short breaks in tone Odbm is defined as lmW through a 600 ohm load ______________________________________
To comply with the domestically applicable LSSGR standards set forth in Table 2 above, or the internationally supported ITU/CCITT recommendations, high quality tone detection is required from a tone detector. To support the capacities needed today, while complying with standards, the approach has been to add processing power through the use of more expensive DSP processors or through the use of multiple DSP processors. Since minimizing system cost is also a primary objective, it is desirable to improve the accuracy and speed of tone detection and verification in a simple and efficient manner.